General Register organization

  Generally CPU has seven general registers.   Register organization show how registers are selected and how data flow between register and ALU.   A decoder is used to select a particular register.The output of each register is connected to two multiplexers to form the two buses A and B. The selection lines in each multiplexer select the input data for the particular bus. 

The A and B buses form the two inputs of an ALU.The operation select lines decide the micro operation to be performed by ALU. The result of the micro operation is available at the output bus. The output bus connected to the inputs of all registers, thus by selecting a destination register it is possible to store the result in it.

A bus organization for seven CPU register

EXAMPLE:

•         To perform the operation R3 = R1+R2  We have to provide following binary selection variable to the select inputs.

1.      SEL A :  001 -To place the contents of R1 into bus A.

2.      SEL B :  010 - to place the contents of R2 into bus B

3.      SEL OPR :  10010  – to perform the arithmetic addition A+B

4.      SEL REG or SEL D :  011  – to place the result available on output bus in R3.

  

Register and multiplexer input selection code

Binary code	SELA	SELB	SELD or SELREG
000	Input	Input	---
001	R1	R1	R1
010	R2	R2	R2
011	R3	R3	R3
100	R4	R4	R4
101	R5	R5	R5
110	R6	R6	R6
111	R7	R7	R7

  

Operation with symbol

                         
  

Operation selection code	Operation	symbol
0000	Transfer A	TSFA
0001	Increment A	INC A
0010	A+B	ADD
0011	A-B	SUB
0100	Decrement A	DEC
0101	A AND B	AND
0110	A OR B	OR
0111	A XOR B	XOR
1000	Complement A	COMA
1001	Shift right A	SHR
1010	Shift left A	SHL

  

What is CONTROL WORD?

•         The combined value of a binary selection inputs specifies the control word.

•         It consist of four fields SELA,SELB,and SELD or SELREG contains three bit each and SELOPR field contains four bits thus the total bits in the control word are 13-bits.

SEL A

SELB

SELREG OR SELD

SELOPR

FORMATE OF CONTROL WORD

1.      The three bit of SELA select a source registers of the a input of the ALU.

2.      The three bits of SELB select a source registers of the b input of the ALU.

3.      The three bits of SELED or SELREG select a destination register using the decoder.

4.      The four bits of SELOPR select the operation to be performed by ALU.

                                CONTROL WORD FOR OPERATION R2 = R1+R3

SEL A	SEL B	SEL D OR SELREG	SELOPR
001	011	010	0010

Note:   Control words for all micro operation are stored in the control memory

Example:

MICROOPERATION	SEL A	SEL B	SEL D OR SELREG	SELOPR	CONTROL WORD
R2 = R1+R3	R1	R3	R2	ADD	001	011	010	0010